The invention of low loss optical fibers in the early 1970s, for use as a practical optical transmission medium, stimulated explosive growth in other areas relating to optical communications. So, for example, subsequent to the development of optical fibers significant effort was directed toward the development of various optical sources and detectors. The concomitant growth of semiconductor technology led to the development of integrated sources and/or detectors which could be easily and inexpensively fabricated. (The term "optical", as used in this application, refers not only to visible light but to any electromagnetic radiation which can be transmitted effectively within dielectric fibers, usually with losses less than 2 dB/kilometer. Accordingly, the term refers to electromagnetic radiation generally of wavelength between 0.1 and 50 microns.)
Along with advances in device development, various system architectures, for use in optical communication systems, have been proposed and continue to be debated. However, many such systems require a light source which is modulated in a manner representative of intelligence. For contemplated long haul systems, such light sources must be pulsed as rapidly as billions of times per second (gigabits/sec). The pulsation may be in the form of an amplitude pulsation, e.g., "on" and "off" states, or a frequency pulsation such as, for example, frequency shift keying in which light of one frequency represents the "on" state and light of another frequency represents the "off" state. Contemplated optical sources must be considered with a view towards their ability to pulse in such a fashion at gigabit rates.
While it is possible to fabricate light sources such as injection lasers which may be inherently pulsed at gigabit rates by direct current modulation, such high pulse rates introduce deleterious spectral-broadening side effects such as "chirp" (T. L. Koch, J. E. Bowers, Electron. Lett., 20, 1038 (1984)). However, such deleterious effects can be reduced if an external modulator is used to vary the otherwise constant output of a light source, or an intracavity modulator is used in a laser, thereby yielding pulsating light representative of intelligence.
In other wavelength division multiplexed ("WDM") applications, it is contemplated that a number of different wavelength channels will be used in optical transmission or switching architectures. For these applications tunable light sources may be used, or tunable optical filtering devices might be used, to determine which wavelength channel is transmitted or received. Such devices can also be used to re-route a given signal within an optical communications network. To achieve such tunability or wavelength channel selection, most devices contemplated employ media whose index of refraction can be modulated or controlled within the device to a desired value.
Particularly useful for modulation as described in the foregoing applications are electrooptic devices whose optical properties, such as absorption or index of refraction, may be varied by application of an appropriate electrical signal. Exemplary of such electrooptical devices is the quantum well device. (In this specification the term "quantum well" refers to one or more quantum wells.)
The quantum well comprises one or more alternating layers of different semiconductor material. The layers alternate between wide bandgap material and narrow bandgap material. The valence band of the wide bandgap material is lower in energy than the valence band of the narrow bandgap material, while the conduction band of the wide bandgap material is higher in energy than the conduction band of the narrow bandgap material. The electrons and holes that are formed in the "well" regions, or that migrate to those regions, are confined to the well regions due to the lower potential energy in these regions. Such devices are called quantum well devices because for narrow wells, the electron and hole energy levels are altered by quantum effects. In the case of excitonic states, confinement of the electrons and holes within a thickness, defined by the narrow bandgap material layer thickness, that is much less than the normal exciton diameter, makes the exciton binding energy larger without further increasing the phonon broadening. This, and other consequences of this "quantum confinement" explains the persistence of the associated resonances to room temperature. In addition, the energies of the confined electrons and holes are increased as a result of the "confinement energy". One incidental consequence of the quantum confinement is that it removes the degeneracy in the valence bonds of the semiconductor, resulting in two exciton resonances, the "light hole" and the "heavy hole" exciton.
When an electric field is applied perpendicular to the quantum well layers, the optical absorption edge, including the exciton resonances, moves to lower photon energies. Normal bulk semiconductors show very little, if any, shift in absorption edge. The only consequence of applying an electric fields to a normal bulk semiconductor is the Franz-Keldysh effect which broadens the band edge with comparatively little shift. At low fields the exciton peaks broaden and disappear. However, unlike the behavior of a bulk semiconductor, when perpendicular fields are applied to a quantum well device the exciton absorption peak remains resolved to high fields.
The preservation of the exciton resonances when perpendicular fields are applied to MQW devices can be explained by considering the effect of an electrical field on a confined electron hole pair. Normally, the application of a field results in exciton broadening because of a shortening of an exciton lifetime due to ionization. However, since the confinement of the electron hole pairs due to the quantum wells precludes exciton ionization, very large fields can be applied without ionization, and therefore without broadening of the exciton resonances. Additionally, and perhaps more importantly, when the MQW device is considered for use as a modulator, there is a significant shift in the absorption edge due to the change in the confinement energy associated with the application of the electric field and the consequent distortion of the well. This shift in absorption is the basis for the MQW as a modulator. Since varying the applied field can significantly alter the light absorption properties of a properly biased MQW, light passing through the MQW will be modulated.
While the electrooptic properties of the MQW described above make it particularly appealing as an optical intensity modulator, the high degree of absorption encountered in the resonance region results in significant loss of optical energy in both the "on" and the "off" states of the modulator, which is most often undesirable, and in some applications may be untolerable. Motivated by this concern various workers have investigated the use of an MQW device as an "index" or "phase" modulator rather than an absorption modulator. The basic Kramers-Kronig relationship, which governs the interplay between the change in a material's absorption characteristics and the change in its index of refraction, dictates that large changes in index of refraction will be experienced in regions removed from the absorption edge. Accordingly, the MQW device may be used as a phase or index modulator with reduced associated absorption losses. Utilization of such an MQW phase modulator in conjunction with a Fabry-Perot laser was suggested in U.S. Pat. No. 4,525,687, issued to Chemla et al., at column 14, lines 1-19.
While early lasers were almost exclusively of the Fabry-Perot type cited by Chemla et al., subsequent developments suggested lasers with distributed reflectors, most often fabricated within a waveguiding structure. U.S. Pat. No. 3,760,292 discusses the operation of a distributed feedback laser. Such a laser is based on the phenomenon of "grating coupling". This phenomenon is associated with the passage of light through a region of varying transmission properties, periodic in at least one dimension, i.e., a "grating". The interaction of light with such gratings involves a phase matching condition which is only satisfied at certain wavelengths. As is well known by those skilled in the art, wavelengths for which the phase matching condition is satisfied are given by the different "orders" of grating coupling. When coupling occurs at the longest wavelength .lambda..sub.0 which satisfies the phase matching condition, the coupling is referred to as "first order". Other wavelengths which may also satisfy the phase matching condition, are .lambda..sub.0 /M where M=2, 3, 4 . . . are the higher orders of operation. In this application the grating is generally considered to be operating in "first order" (M=1) but may operate in higher orders. Representative of such "gratings" are regions of varying index of refraction or a corrugation in the boundary between two materials of different index of refraction. When light passes through such a region it may be either forward or reverse coupled to other guided waves. Exemplary of such "grating coupling" is the Bragg reflector--a "grating coupler" which reverse couples incoming light to a "reflected", outgoing, light beam. The Bragg reflector may be fabricated external to a gain medium resulting in a Distributed Bragg Reflecting (DBR) laser. A "grating coupler" may also be fabricated as an integral part of the gain material resulting in a Distributed Feed Back (DFB) laser. The grating coupler in a DFB laser may be viewed heuristically as simply a reflector which defines the laser cavity. However, rigorous analysis shows that the coupling in a DFB laser is more complicated because of phase effects. So, for example, the DFB does not lase at the center of the "Bragg reflection band" but rather near its two first minima.
The quantum well device and the distributed feedback laser have been combined to yield devices such as quantum well lasers. The quantum well laser utilizes the electronic properties of the quantum well to define energy levels which are particularly advantageous for use as laser transitions.